This invention relates generally to synchronous communications systems and is particularly directed to establishing and maintaining synchronization of a synchronous serial data receiver with transmitted serial data.
Synchronization between transmitter and receiver is required in most communications systems. For example, the signal driving a video display is comprised of a video component and a synchronization component. While the video signal portion includes picture information such as luminance and chrominance signals, the synchronization signal portion includes pulse-like signals occurring both at the horizontal and vertical scan rates which are interspersed between the scan interval of the picture component for synchronizing video display scan with the receipt of the video signals. In another example, downstream signals in a cable television (CATV) system include a video signal portion as well as subscriber identity and program authorization information. The latter information is typically provided in the transmitted signal during the vertical retrace interval and the subscriber terminal must therefore not only be capable of synchronizing video display sweep with received video information, but also of synchronizing the subscriber decoder with the received subscriber identity and program authorization data.
In synchronous communications systems, synchronization between transmitter and receiver is generally provided by means of two alternative approaches. In one approach, the transmitter and receiver are each individually driven by separate clocks relative to a mutual time reference. This requires highly accurate timing provided by generally expensive timing devices in both the transmitter and the receiver. More commonly, receiver timing is established by received signal timing. The aforementioned video signal sync pulse is an example of this. In non-video related oommunications systems the received signal may include a sync pulse, or pulses, uniquely positioned in the received data bit stream to provide receiver synchronization.
Generally, once synchronization is lost, a synchronous signal receiver must be reset, or initialized, to a set of initial conditions before synchronization can be restored. This is particularly true in the case of digital signal receivers which frequently are under the control of a microprocessor or micro-computer. In these systems, the sequence of operations in the receiver must be initialized to a predetermined location in the operating program for proper receipt and processing of the transmitted data. To date, attempts to re-establish receiver synchronization have either required manual intervention by an operator in resetting the receiver to a predetermined set of initial conditions, or have involved complicated and expensive fault detection arrangements which are generally unable to rapidly restore synchronization and thus suffer from communication link drop-outs and an associated loss of data.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a data error reset system for a synchronous signal receiver which is responsive to groups of timing bits, or characters, in the received serial data for determining if the receiver is synchronized to serial data and if the receiver is not synchronized, for automatically re-initializing the receiver to a predetermined operating state for re-establishing synchronization with data from a synchronous serial transmitter.